Differential regulation of midbrain dopaminergic neuron development by Wnt-1, Wnt-3a, and Wnt-5a

Function of Wnts in Dopaminergic Neuron Development

Dopaminergic (DA) neurons in the ventral midbrain (VM) are one of the major cell types lost in Parkinson's disease (PD). Proof of principle exists for cell replacement therapies for PD, but wider application is halted by the unavailability of abundant sources of DA neurons. Stem cells might constitute one of these sources. However, efficient protocols promoting their specific differentiation into a DA neuronal phenotype are required. In this review, we summarize the latest findings concerning the contribution of the Wnt family of glycolipoproteins in the development of VM DA neurons. Regulators of Wnt signaling are involved in several neurodevelopmental processes. Recent results indicate that Wnts are key regulators of proliferation and differentiation of DA precursors during VM neurogenesis and different Wnts have specific and unique activity profiles. Interestingly, chemical inhibitors of glycogen synthase kinase-3beta stabilize beta-catenin and increase DA differentiation in VM precursor cultures. We hereby propose that Wnts are likely to contribute in the future to improve stem/precursor cell replacement therapy approaches to PD.

Dynamic temporal and cell type-specific expression of Wnt signaling components in the developing midbrain

Wnt1 and -5a have been shown to modulate the proliferation and differentiation of midbrain dopaminergic (DA) neurons. However, it is not known whether other Wnts or which Frizzled (Fz) receptors are expressed in the developing midbrain. We found that 13 out of 19 Wnts, all 10 Fzs, and several intracellular Wnt signaling modulators, including Axin, FRAT, Naked, Par-1, and Ltap are developmentally regulated between embryonic days (E) 10.5 and 15.5. Next, we studied whether Fzs are differentially expressed in different cell types and examined neuronal-progenitor- or glial-enriched cultures and DA neurons isolated from TH-GFP reporter mice. We found that Fz8 is expressed at high levels in DA neurons at E11.5 and E13.5. Fz6 and -7 are the predominant transcripts in glial precursors, and Fz9, which is absent in DA neurons at E11.5, is the main receptor expressed in neuronal precursors. We therefore examined the function of Fz9 in DA cells and found that overexpression of Fz9 reduced Wnt5a- but not Wnt3a-induced hyperphosphorylation of Dishevelled. Thus, our results show that Fzs are developmentally regulated and differentially expressed in VM precursors, DA neurons, and glia. These findings suggest that Fz expression contributes to provide specificity to Wnt-mediated effects.

Treatment with deferoxamine increases neurons from neural stem/progenitor cells

Neural transplantation is a promising approach for treating neurodegenerative disease. Neural stem/progenitor cells (NPCs) are self-renewing and multipotent and thus are good candidates for donor cells when they have been clearly defined to differentiate into neurons. As neuronal differentiation follows cell cycle exit, we investigated whether neuron production from NPCs is increased by treatment with cell cycle blockers. NPCs from E12.5 rat ventral mesencephalon were cultured as neurospheres in DMEM/F12 medium containing N2 supplements and bFGF. Treatment of NPCs with deferoxamine, a G1/S phase blocker, increased the number of beta-tubulin III-positive cells after differentiation, concomitant with increases of MAP2 mRNA and protein, and a decrease of GFAP protein. Further, an increase in beta-tubulin III/BrdU double-positive cells and a decrease in GFAP/BrdU double-positive cells were confirmed. In real-time PCR, the expressions of p21(cip1), p27(kip1) and p57(kip2) mRNAs remained unaltered for 8 h after treatment with deferoxamine but were significantly elevated after 1 day. Deferoxamine specifically enhanced the elevation of p27(kip1) mRNA at 1-2 days and the accumulation of p27(kip1) protein at 3 days, along with the activation of neuroD promoter and the elevation of neuroD mRNA. Transfection of p27(kip1) into NPCs induced activation of neuroD promoter and increase of number of beta-tubulin III-positive cells. These data suggest that pretreatment with deferoxamine increases the number of neurons from NPCs related to prolonged p27(kip1) elevation and activation of the neuroD signaling pathway. In this way, regulation of the cell cycle should be a useful first step in engineering NPCs for neural transplantation.

Non-fibrillar β-amyloid abates spike-timing-dependent synaptic potentiation at excitatory synapses in layer 2/3 of the neocortex by targeting postsynaptic AMPA receptors

Cognitive decline in Alzheimer's disease (AD) stems from the progressive dysfunction of synaptic connections within cortical neuronal microcircuits. Recently, soluble amyloid beta protein oligomers (Abeta(ol)s) have been identified as critical triggers for early synaptic disorganization. However, it remains unknown whether a deficit of Hebbian-related synaptic plasticity occurs during the early phase of AD. Therefore, we studied whether age-dependent Abeta accumulation affects the induction of spike-timing-dependent synaptic potentiation at excitatory synapses on neocortical layer 2/3 (L2/3) pyramidal cells in the APPswe/PS1dE9 transgenic mouse model of AD. Synaptic potentiation at excitatory synapses onto L2/3 pyramidal cells was significantly reduced at the onset of Abeta pathology and was virtually absent in mice with advanced Abeta burden. A decreased alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/N-methyl-D-aspartate (NMDA) receptor-mediated current ratio implicated postsynaptic mechanisms underlying Abeta synaptotoxicity. The integral role of Abeta(ol)s in these processes was verified by showing that pretreatment of cortical slices with Abeta((25-35)ol)s disrupted spike-timing-dependent synaptic potentiation at unitary connections between L2/3 pyramidal cells, and reduced the amplitude of miniature excitatory postsynaptic currents therein. A robust decrement of AMPA, but not NMDA, receptor-mediated currents in nucleated patches from L2/3 pyramidal cells confirmed that Abeta(ol)s perturb basal glutamatergic synaptic transmission by affecting postsynaptic AMPA receptors. Inhibition of AMPA receptor desensitization by cyclothiazide significantly increased the amplitude of excitatory postsynaptic potentials evoked by afferent stimulation, and rescued synaptic plasticity even in mice with pronounced Abeta pathology. We propose that soluble Abeta(ol)s trigger the diminution of synaptic plasticity in neocortical pyramidal cell networks during early stages of AD pathogenesis by preferentially targeting postsynaptic AMPA receptors.

Increase in proliferation and differentiation of neural progenitor cells isolated from postnatal and adult mice brain by Wnt-3a and Wnt-5a

Wnt signaling is implicated in the control of cell growth and differentiation during CNS development. These findings are based on studies of mouse and chick models. However, the action of Wnt signaling, at the cellular level, is poorly understood. In this study, we investigated the roles of Wnt-3a and Wnt-5a on differentiation and proliferation of postnatal neural progenitor cells (NPCs) in mice.NPCs were isolated from the subventricular zone (SVZ) of PN-1 and adult ICR mice. Plasmids containing active Wnt-3a or Wnt-5a were transfected to NPCs; their effects on the formation of neurospheres and differentiation into neuronal cells were then determined. Transfection of Wnt-3a and Wnt-5a plasmids promoted regeneration of neurospheres and differentiation into Map2-positive cells, and decreased differentiation into GFAP-positive cells. The conditioned media obtained from Wnt-3a or Wnt-5a transfected NPCs showed similar effects on differentiation of NPCs with cDNA transfection, although the magnitude of stimulatory effect was less than that by plasmid transfection. Wnt-3a and Wnt-5a transfection did not affect Brdu incorporation of neuronal or glial progenitors in differentiation media. Wnt-3a and Wnt-5a plasmid transfection and the treatment of Wnt-3a and Wnt-5a conditioned media increased beta-catenin levels in NPCs. Wnt-3a had a greater effect on beta-catenin levels than Wnt-5a. The PKC inhibitor completely blocked the Wnt-5a effect on neuronal differentiation in NPCs. These findings suggest that Wnt-3a and Wnt-5a each have distinct effects on the proliferation and differentiation of NPCs in postnatal mice.

Neurogenin 2 is required for the development of ventral midbrain dopaminergic neurons

Proneural genes are crucial regulators of neurogenesis and subtype specification in many areas of the nervous system; however, their function in dopaminergic neuron development is unknown. We report that proneural genes have an intricate pattern of expression in the ventricular zone of the ventral midbrain, where mesencephalic dopaminergic neurons are generated. Neurogenin 2(Ngn2) and Mash1 are expressed in the ventral midline, while Ngn1, Ngn2 and Mash1 are co-localized more laterally in the ventricular zone. Ngn2 is also expressed in an intermediate zone immediately adjacent to the ventricular zone at the ventral midline. To examine the function of these genes, we analyzed mutant mice in which one or two of these genes were deleted (Ngn1, Ngn2 and Mash1) or substituted (Mash1 in the Ngn2 locus). Our results demonstrate that Ngn2 is required for the differentiation of Sox2+ ventricular zone progenitors into Nurr1+postmitotic dopaminergic neuron precursors in the intermediate zone, and that it is also likely to be required for their subsequent differentiation into tyrosine hydroxylase-positive dopaminergic neurons in the marginal zone. Although Mash1 normally has no detectable function in dopaminergic neuron development, it could partially rescue the generation of dopaminergic neuron precursors in the absence of Ngn2. These results demonstrate that Ngn2 is uniquely required for the development of midbrain dopaminergic neurons.

Ventral midbrain glia express region-specific transcription factors and regulate dopaminergic neurogenesis through Wnt-5a secretion

Glial cells have been classically described as supporting cells for neurons. Recently, additional roles during neural development have begun to emerge. Here, we report that ventral midbrain glia, including astrocytes and radial glia, are the source of signals required by neural precursors to acquire a dopaminergic phenotype. We found that ventral midbrain glia, but not cortical glia, secrete high levels of the glycolipoprotein Wnt-5a, express region-specific transcription factors such as Pax-2, En-1 and Otx-2 and increase the differentiation of cortical or ventral midbrain Nurr1 precursors into tyrosine hydroxylase-positive neurons. Moreover, blocking experiments using a Wnt-5a blocking antibody indicated that the effects of ventral midbrain glia on Nurr1-positive neural precursors are partially mediated by Wnt-5a. Thus, our results identify Wnt-5a as an important component of the dopaminergic inductive activity of the ventral midbrain glia.

Cripto as a target for improving embryonic stem cell-based therapy in Parkinson's disease

Embryonic stem (ES) cells have been suggested as candidate therapeutic tools for cell replacement therapy in neurodegenerative disorders. However, limitations for the use of these cells lie in our restricted knowledge of the molecular mechanisms involved in their specialized differentiation and in the risk of tumor formation. Recent findings suggest that the EGF-CFC protein Cripto is a key player in the signaling pathways controlling neural induction in ES cells. Here we show that in vitro differentiation of Cripto(-/-) ES cells results in increased dopaminergic differentiation and that, upon transplantation into Parkinsonian rats, they result in behavioral and anatomical recovery with no tumor formation. The use of knockout ES cells that can generate dopamine cells while eliminating tumor risk holds enormous potential for cell replacement therapy in Parkinson's disease.

Engineering a Dopaminergic Phenotype in Stem/Precursor Cells: Role of Nurr1, Glia-Derived Signals, and Wnts

Recent results from clinical trials using fetal tissue grafts in patients with Parkinson's disease (PD) have indicated that current surgical strategies for dopamine cell replacement therapy need to be improved in order to achieve better functional integration of the grafts and to avoid dyskinesias. Previous studies using rich dopaminergic (DA) cell suspensions have provided proof-of-concept that PD patients can benefit from cell replacement therapy. Stem cells have been proposed as better candidates for cell replacement therapy in PD since they can be standardized, expanded, and engineered in vitro. Recent developments indicate that cell preparations enriched in DA neurons can be generated in vitro, but their functional integration in animal models of disease is still far from optimal. This is not entirely surprising considering our limited knowledge of the development of DA neurons and the reduced number of factors that have been implemented in stem cell differentiation protocols. This review will focus on three aspects of DA neuron development: (1) the function of Nurr1 and retinoid X receptors (RXR) in the differentiation of DA precursors and in the survival of DA neurons; (2) the role of glia in DA neurogenesis and the differentiation of DA precursors; and (3) the function of the Wnt family of lipoproteins in the proliferation and differentiation of DA precursors. A greater understanding of the cellular and molecular mechanisms that control DA neuron development, as well as their functional integration in vivo, are likely to ultimately contribute to the development of novel stem cell replacement therapies for PD.

Winding through the WNT pathway during cellular development and demise

In slightly over a period of twenty years, our comprehension of the cellular and molecular mechanisms that govern the Wnt signaling pathway continue to unfold. The Wnt proteins were initially implicated in viral carcinogenesis experiments associated with mammary tumors, but since this period investigations focusing on the Wnt pathways and their transmembrane receptors termed Frizzled have been advanced to demonstrate the critical nature of Wnt for the development of a variety of cell populations as well as the potential of the Wnt pathway to avert apoptotic injury. In particular, Wnt signaling plays a significant role in both the cardiovascular and nervous systems during embryonic cell patterning, proliferation, differentiation, and orientation. Furthermore, modulation of Wnt signaling under specific cellular influences can either promote or prevent the early and late stages of apoptotic cellular injury in neurons, endothelial cells, vascular smooth muscle cells, and cardiomyocytes. A number of downstream signal transduction pathways can mediate the biological response of the Wnt proteins that include Dishevelled, beta-catenin, intracellular calcium, protein kinase C, Akt, and glycogen synthase kinase-3beta. Interestingly, these cellular cascades of the Wnt-Frizzled pathways can participate in several neurodegenerative, vascular, and cardiac disorders and may be closely integrated with the function of trophic factors. Identification of the critical elements that modulate the Wnt-Frizzled signaling pathway should continue to unlock the potential of Wnt pathway for the development of new therapeutic options against neurodegenerative and vascular diseases.

Developmental origin and fate of meso-diencephalic dopamine neurons

Specific vulnerability of substantia nigra compacta neurons as compared to ventral tegmental area neurons, as emphasized in Parkinson's disease, has been studied for many years and is still not well understood. The molecular codes and mechanisms that drive development of these structures have recently been studied through the use of elegant genetic ablation experiments. The data suggested that specific genes at specific anatomical positions in the ventricular zone are crucial to drive development of young neurons into the direction of the dopaminergic phenotype. In addition, it has become clear the these dopaminergic neurons are present in the diencephalon and in the mesencephalon and that they may contain a specific molecular signature that defines specific subsets in terms of position and function. The data indicate that these specific subsets may explain the specific response of these neurons to toxins and genetic ablation.

Generation of graftable dopaminergic neuron progenitors from mouse ES cells by a combination of coculture and neurosphere methods

Parkinson's disease is characterized by a loss of midbrain dopamine (DA) neurons and is generally viewed as a potential target for stem cell therapy. Although several studies have reported the generation of postmitotic DA neurons from embryonic stem (ES) cells, it is unknown whether the proliferative progenitors of DA neurons can be isolated in vitro. To investigate this possibility, we have developed a combined approach in which ES cells are cocultured with PA6 stromal cells to expose them to stromal cell-derived inducing activity (SDIA) and are then cultured as neurospheres. Mouse ES cell colonies were detached from PA6 feeder cells after 8 days of SDIA treatment and then expanded as spheres for another 4 days in serum-free medium supplemented with fibroblast growth factor-2. The spheres exhibited neural stem cell characteristics and contained few DA neurons at this stage of culture. After being induced to differentiate on polyornithine/laminin-coated dishes for 7 days, these spheres generated DA neurons in vitro at a relatively low frequency. Intriguingly, addition of PA6 cell conditioned medium to the sphere culture medium significantly increased the percentage of DA neurons to 25-30% of the total number of neurons. Transplantation of conditioned medium-treated day 4 spheres, which contained DA neuron progenitors, into the mouse striatum resulted in the generation of a significant number of graft-derived DA neurons. These findings suggest that progenitors of DA neurons are generated and can proliferate in ES cell-derived neurospheres induced by serial SDIA and PA6 conditioned medium treatment.

Endocannabinoids regulate interneuron migration and morphogenesis by transactivating the TrkB receptor

In utero exposure to Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the active component from marijuana, induces cognitive deficits enduring into adulthood. Although changes in synaptic structure and plasticity may underlie Delta(9)-THC-induced cognitive impairments, the neuronal basis of Delta(9)-THC-related developmental deficits remains unknown. Using a Boyden chamber assay, we show that agonist stimulation of the CB(1) cannabinoid receptor (CB(1)R) on cholecystokinin-expressing interneurons induces chemotaxis that is additive with brain-derived neurotrophic factor (BDNF)-induced interneuron migration. We find that Src kinase-dependent TrkB receptor transactivation mediates endocannabinoid (eCB)-induced chemotaxis in the absence of BDNF. Simultaneously, eCBs suppress the BDNF-dependent morphogenesis of interneurons, and this suppression is abolished by Src kinase inhibition in vitro. Because sustained prenatal Delta(9)-THC stimulation of CB(1)Rs selectively increases the density of cholecystokinin-expressing interneurons in the hippocampus in vivo, we conclude that prenatal CB(1)R activity governs proper interneuron placement and integration during corticogenesis. Moreover, eCBs use TrkB receptor-dependent signaling pathways to regulate subtype-selective interneuron migration and specification.

A focused microarray to assess dopaminergic and glial cell differentiation from fetal tissue or embryonic stem cells

We designed oligonucleotide gene-specific probes to develop a focused array that can be used to discriminate between neural phenotypes, identify biomarkers, and provide an overview of the process of dopaminergic neuron and glial differentiation. We have arrayed approximately 100 genes expressed in dopaminergic neurons, oligodendrocytes, and astrocytes, an additional 200 known cytokines, chemokines, and their respective receptors, as well as markers for pluripotent and progenitor cells. The gene-specific 60-mer 3' biased oligonucleotides for these 281 genes were arrayed in a 25 x 12 format based on function. Using human adult brain substantia nigra, human embryonic stem cells (ESCs), and the differentiated progeny of pluripotent cells, we showed that this array was capable of distinguishing dopaminergic neurons, glial cells, and pluripotent cells by their gene expression profiles in a concentration-dependent manner. Using linear correlation coefficients of input RNA with output intensity, we identified a list of genes that can serve as reporting genes for detecting dopaminergic neurons, glial cells, and contaminating ESCs and progenitors. Finally, we monitored NTera2 differentiation toward dopaminergic neurons and have shown the ability of this array to distinguish stages of differentiation and provide important clues to factors regulating differentiation, the degree of contaminating populations, and stage of cell maturity. We suggest that this focused array will serve as a useful complement to other large-scale arrays in routine assessment of cell properties prior to their therapeutic use.

Wnt-5a gene expression in malignant human neuroblasts

Neuroblastoma (NB), an embryonal malignancy, poses a major challenge in pediatric oncology for the treatment of disseminated forms. Here, we report the decrease of Wnt-5a gene expression in high-risk NB (HR-NB) as well as in cultured metastatic neuroblasts. Wnt-5a is a member of the Wnt signaling pathway which is mainly associated with patterning decisions in the embryonic nervous system. Moreover, Wnt-5a has been involved in metastatic melanoma progression and invasive ductal breast cancer via adhesion and migration alterations. As retinoic acid (RA) plays a major role in the neural crest induction and differentiation, we showed that RA reverses the aberrant negative regulation of Wnt-5a in metastatic neuroblasts. While beta-catenin expression remained unchanged, PKC-theta, a protein kinase C isoform, was evidenced to increase and parallel Wnt-5a level. For the first time, the involvement of Wnt-5a through the Wnt/calcium signaling is highlighted in the pathogenesis of a pediatric embryonal malignancy, NB.

The role of Nurr1 in the development of dopaminergic neurons and Parkinson's disease

Nurr1, a transcription factor belonging to the orphan nuclear receptor superfamily, is critical in the development and maintenance of the dopaminergic system and as such it may have role in the pathogenesis of Parkinson' disease (PD). Human Nurr1 gene has been mapped to chromosome 2q22-23 and Nurr1 protein is predominantly expressed in central dopaminergic neurons. Nurr1 interacts with other factors critical for the survival of mensencephalic dopaminergic neurons and it appears to regulate the expression of tyrosine hydroxylase (TH), dopamine transporter (DAT), vesicular monoamine transporter 2 (VMAT2), and l-aromatic amino acid decarboxylase (AADC), all of which are important in the synthesis and storage of dopamine. Experimental studies in Nurr1 knock-out mice indicate that Nurr1 deficiency results in impaired dopaminergic function and increased vulnerability of those midbrain dopaminergic neurons that degenerate in PD. Decreased Nurr1 expression is found in the autopsied PD midbrains, particularly in neurons containing Lewy bodies, as well as in peripheral lymphocytes of patients with parkinsonian disorders. Several variants in Nurr1 gene have been reported in association with PD. All these studies suggest that Nurr1 is not only essential in the development of mensencephalic dopaminergic neurons and maintenance of their functions, but it may also play a role in the pathogenesis of PD.

Increased Wnt levels in the neural tube impair the function of adherens junctions during neurulation

Wnt7a has been reported to signal via the canonical pathway, but also in non-canonical pathways acting on the cytoskeleton. Since Wnt7a is expressed after neurulation, we set to investigate the effects of Wnt7a on brain regionalization. We engineered transgenic mouse embryos that, under control of the nestin second intron, overexpressed Wnt7a in neural stem/progenitor cells. Surprisingly, transgenic embryos failed to complete cranial neurulation due to reduced levels and an impaired distribution of actin microfilaments, beta-catenin, and N-cadherin at the neural tube adherens junctions. These transgenic embryos expressed high levels of Vangl2, an essential component of non-canonical Wnt signaling. In agreement with a disregulation of this pathway, aberrant spinal neurulation was detected in the transgenic embryos, revealing a novel function regulated by Wnts. Thus, our findings suggest that Wnt7a overexpression disrupts normal Wnt signaling in the neural tube, resulting in defective adherens junctions and neurulation.

The effects of antipsychotics on beta-catenin, glycogen synthase kinase-3 and dishevelled in the ventral midbrain of rats

Protein kinase B and glycogen synthase kinase-3 have been identified as susceptibility genes for schizophrenia and altered protein and mRNA levels have been detected in the brains of schizophrenics post-mortem. Recently, we reported that haloperidol, clozapine and risperidone alter glycogen synthase kinase-3 and beta-catenin protein expression and glycogen synthase kinase-3 phosphorylation levels in the rat prefrontal cortex and striatum. In the current study, beta-catenin, adenomatous polyposis coli, Wnt1, dishevelled and glycogen synthase kinase-3 were examined in the ventral midbrain and hippocampus using western blotting. In addition, beta-catenin and GSK-3 were examined in the substantia nigra and ventral tegmental area using confocal and fluorescence microscopy. The results indicate that repeated antipsychotic administration results in significant elevations in glycogen synthase kinase-3, beta-catenin and dishevelled-3 protein levels in the ventral midbrain and hippocampus. Raclopride causes similar changes in beta-catenin and GSK-3 in the ventral midbrain, suggesting that D2 dopamine receptor antagonism mediated the changes observed following antipsychotic administration. In contrast, amphetamine, a drug capable of inducing psychotic episodes, had the opposite effect on beta-catenin and GSK-3 in the ventral midbrain. Collectively, the results suggest that antipsychotics may exert their beneficial effects through modifications to proteins that are associated with the canonical Wnt pathway.

Midbrain dopaminergic development in vivo and in vitro from embryonic stem cells

The midbrain dopaminergic (mDA) neurons play a key role in the function of a variety of brain systems, including motor control and reward pathways. This has led to much interest in these neurons as targets for intervention in human disorders such as Parkinson's disease and schizophrenia. A major area of interest is to direct embryonic stem (ES) cells to differentiate into mDA neurons in vitro, which can then be used for cell therapy or drug screening. At present, our understanding of mDA development in vivo is limited. However, recent studies have identified a number of regulatory factors that influence the development of mDA neurons in vivo. Such studies will not only increase our understanding of mDA development in vivo, they may also promote new paradigms for regulating mDA production from ES cells in vitro. Here we review the current knowledge on mDA development in vivo and mDA differentiation.

Activation of the PI3-K/Akt pathway mediates cGMP enhanced-neurogenesis in the adult progenitor cells derived from the subventricular zone

The intracellular mechanisms that regulate neurogenesis remain unclear. Using neurospheres isolated from the subventricular zone (SVZ) of the adult rat, we investigated the effect of cyclic guanosine monophosphate (cGMP) and its signaling pathway on the induction of neurogenesis. Neurospheres expressed phosphodiesterase 5 (PDE5) and treatment of neurospheres with Sildenafil, a specific inhibitor of PDE5, significantly increased cGMP levels and neurogenesis. In addition, incubation of neurospheres with Sildenafil significantly phosphorylated Akt, which was associated with an increase of phosphorylation of glycogen synthase kinase 3 (GSK-3), a downstream target of Akt. Coincubation of neurospheres with Sildenafil and LY 294002, a pharmacological inhibitor of PI3-K/Akt, abolished Sildenafil-induced phosphorylated Akt and GSK-3. Furthermore, LY 294002 blocked Sildenafil-increased SVZ cell proliferation. These data suggest that Sildenafil-enhanced neurogenesis likely occurs through activation of the PI3-K/Akt/GSK-3 pathway.

Nodal-dependant Cripto signaling in ES cells: from stem cells to tumor biology

Embryonic stem (ES) cells have provided a valid model to understand early events of mammalian lineage specification and differentiation, leading to important insights into the mechanisms that control embryogenesis at the molecular and cellular levels. Furthermore, ES cells have recently evoked great scientific interest as ideal candidates for the generation of tissues for transplantation therapies. In this respect, particular attention has been paid to the molecules and signaling pathways triggering ES cell differentiation. The EGF-CFC Cripto protein is a key regulator of ES cells fate. The cripto gene is expressed both in ES cells and during the early phases of embryo development, while, in the adult, it is reactivated in a wide range of epithelial cancers. This review will discuss recent findings on the molecular basis of Cripto signaling in ES cell differentiation, providing an intriguing link between stem cell and tumor biology.

Hippocampal and visuospatial learning defects in mice with a deletion of frizzled 9, a gene in the Williams syndrome deletion interval

Wnt signaling regulates hippocampal development but little is known about the functions of specific Wnt receptors in this structure. Frizzled 9 is selectively expressed in the hippocampus and is one of about 20 genes typically deleted in Williams syndrome. Since Williams syndrome is associated with severe visuospatial processing defects, we generated a targeted null allele for frizzled 9 to examine its role in hippocampal development. Frizzled 9-null mice had generally normal gross anatomical hippocampal organization but showed large increases in apoptotic cell death in the developing dentate gyrus. This increase in programmed cell death commenced with the onset of dentate gyrus development and persisted into the first postnatal week of life. There was also a perhaps compensatory increase in the number of dividing precursors in the dentate gyrus, which may have been a compensatory response to the increased cell death. These changes in the mutants resulted in a moderate decrease in the number of adult dentate granule cells in null mice and an increase in the number of hilar mossy cells. Heterozygous mice (the same frizzled 9 genotype as Williams syndrome patients) were intermediate between wild type and null mice for all developmental neuronanatomic defects. All mice with a mutant allele had diminished seizure thresholds, and frizzled 9 null mice had severe deficits on tests of visuospatial learning/memory. We conclude that frizzled 9 is a critical determinant of hippocampal development and is very likely to be a contributing factor to the neurodevelopmental and behavioral phenotype of patients with Williams syndrome.

Wnt signaling regulates the sequential onset of neurogenesis and gliogenesis via induction of BMPs

In the mammalian central nervous system, neurogenesis precedes gliogenesis; neurons are primarily generated at the neural stage, whereas most glial cells are generated perinatally and postnatally. However, the signals that regulate this sequence of events remain unclear. Here we show that Wnt signaling induces neuronal and astroglial differentiation but suppresses oligodendroglial differentiation. We observed that precursor cells infected with a retrovirus encoding beta-catenin differentiated into neurons, while astrocytes developed from uninfected precursor cells surrounding infected cells. As neurogenesis proceeded, expression of the bone morphogenetic proteins (BMPs), BMP2, 4 and 7, progressively increased in the cells infected with the retrovirus encoding beta-catenin. Furthermore, treatment of cells with Noggin, a BMP antagonist, completely inhibited astroglial differentiation but partially restored oligodendroglial differentiation. These results suggest that Wnt signaling indirectly regulates gliogenesis by inducing BMPs in neuronal cells. Thus, cooperation between Wnt and BMP signaling may play a key role in determining the sequence of neurogenesis and gliogenesis.

Combination of hTERT and bmi-1, E6, or E7 induces prolongation of the life span of bone marrow stromal cells from an elderly donor without affecting their neurogenic potential

Murine bone marrow stromal cells differentiate not only into mesodermal derivatives, such as osteocytes, chondrocytes, adipocytes, skeletal myocytes, and cardiomyocytes, but also into neuroectodermal cells in vitro. Human bone marrow stromal cells are easy to isolate but difficult to study because of their limited life span. To overcome this problem, we attempted to prolong the life span of bone marrow stromal cells and investigated whether bone marrow stromal cells modified with bmi-1, hTERT, E6, and E7 retained their differentiated capability, or multipotency. In this study, we demonstrated that the life span of bone marrow stromal cells derived from a 91-year-old donor could be extended and that the stromal cells with an extended life span differentiated into neuronal cells in vitro. We examined the neuronally differentiated cells morphologically, physiologically, and biologically and compared the gene profiles of undifferentiated and differentiated cells. The neuronally differentiated cells exhibited characteristics similar to those of midbrain neuronal progenitors. Thus, the results of this study support the possible use of autologous-cell graft systems to treat central nervous system diseases in geriatric patients.

Factors involved in the determination of the neurotransmitter phenotype of developing neurons of the CNS: Applications in cell replacement treatment for Parkinson's disease

The developmental stages involved in the conversion of stem cells to fully functional neurons of specific neurotransmitter phenotype are complex and not fully understood. Over the past decade many studies have been published that demonstrate that in vitro manipulation of the epigenetic environment of the stem cells allows experimental control of final neuronal phenotypic choice. This review presents the evidence for the involvement of a number of endogenous neurobiochemicals, which have been reported to potently influence DAergic (and other neurotransmitter) phenotype expression in vitro. They act at different stages on the pathway to neurotransmitter phenotype determination, and in different ways. Many are better known for their involvement in other aspects of development, and in other biochemical roles. Their proper place, and precise roles, in neurotransmitter phenotype determination in vivo will no doubt be determined in the future. Meanwhile, considerable medical benefits are offered from producing large, long-term, viable cryostores of self-regenerating multipotential neural precursor cells (i.e., brain stem cells), which can be used for cell replacement therapies in the treatment of degenerative brain diseases, such as Parkinson's disease.

A mutually stimulating loop involving emx2 and canonical wnt signalling specifically promotes expansion of occipital cortex and hippocampus

The correct size of the different areas composing the mature cerebral cortex depends on the proper early allocation of cortical progenitors to their distinctive areal fates, as well as on appropriate subsequent tuning of their area-specific proliferation-differentiation profiles. Whereas much is known about the genetics of the former process, the molecular mechanisms regulating proliferation and differentiation rates within distinctive cortical proto-areas are still largely obscure. Here we show that a mutual stimulating loop, involving Emx2 and canonical Wnt signalling, specifically promotes expansion of the occipito-hippocampal anlage. Collapse of this loop occurring in Emx2-/- mutants leads progenitors within this region to slow down DNA synthesis and exit prematurely from the cell cycle, due to misregulation of cell cycle-, proneural- and lateral inhibition-molecular machineries, and eventually results in dramatic and selective size-reduction of occipital cortex and hippocampus. Reactivation of canonical Wnt signalling in the same mutants rescues a subset of molecular abnormalities and corrects differentiation rates of occipito-hippocampal progenitors.

Development of the dopaminergic neurons in the rodent brainstem

The loss of dopaminergic (DA) neurons in the ventral midbrain is the principal cause of Parkinson's disease. The search for candidate molecules that promote the genesis and survival capacities of DA neurons is a major area of investigation and hope. A better characterization of the developmental pathways that govern the specification, differentiation, and survival of these neurons will be essential in devising therapies aimed to rescue or replace midbrain DA neurons in Parkinson's patients. In this brief review, we will discuss the major steps in the normal development of midbrain DA neurons.

Differences in gene expression between wild type and Hoxa1 knockout embryonic stem cells after retinoic acid treatment or leukemia inhibitory factor (LIF) removal

Homeobox (Hox) genes encode a family of transcription factors that regulate embryonic patterning and organogenesis. In embryos, alterations of the normal pattern of Hox gene expression result in homeotic transformations and malformations. Disruption of the Hoxa1 gene, the most 3' member of the Hoxa cluster and a retinoic acid (RA) direct target gene, results in abnormal ossification of the skull, hindbrain, and inner ear deficiencies, and neonatal death. We have generated Hoxa1(-/-) embryonic stem (ES) cells (named Hoxa1-15) from Hoxa1(-/-) mutant blastocysts to study the Hoxa1 signaling pathway. We have characterized in detail these Hoxa1(-/-) ES cells by performing microarray analyses, and by this technique we have identified a number of putative Hoxa-1 target genes, including genes involved in bone development (e.g. Col1a1, Postn/Osf2, and the bone sialoprotein gene or BSP), genes that are expressed in the developing brain (e.g. Nnat, Wnt3a, BDNF, RhoB, and Gbx2), and genes involved in various cellular processes (e.g. M-RAS, Sox17, Cdkn2b, LamA1, Col4a1, Foxa2, Foxq1, Klf5, and Igf2). Cell proliferation assays and Northern blot analyses of a number of ES cell markers (e.g. Rex1, Oct3/4, Fgf4, and Bmp4) suggest that the Hoxa1 protein plays a role in the inhibition of cell proliferation by RA in ES cells. Additionally, Hoxa1(-/-) ES cells express high levels of various endodermal markers, including Gata4 and Dab2, and express much less Fgf5 after leukemia inhibitory factor (LIF) withdrawal. Finally, we propose a model in which the Hoxa1 protein mediates repression of endodermal differentiation while promoting expression of ectodermal and mesodermal characteristics.

Purified Wnt-5a increases differentiation of midbrain dopaminergic cells and dishevelled phosphorylation

The Wnt family of lipoproteins regulates several aspects of the development of the nervous system. Recently, we reported that Wnt-3a enhances the proliferation of midbrain dopaminergic precursors and that Wnt-5a promotes their differentiation into dopaminergic neurones. Here we report the purification of hemagglutinin-tagged Wnt-5a using a three-step purification method similar to that previously described for Wnt-3a. Haemagglutinin-tagged Wnt-5a was biologically active and induced the differentiation of immature primary midbrain precursors into tyrosine hydroxylase-positive dopaminergic neurones. Using a substantia nigra-derived dopaminergic cell line (SN4741), we found that Wnt-5a, unlike Wnt-3a, did not promote beta-catenin phosphorylation or stabilization. However, both Wnt-5a and Wnt-3a activated dishevelled, as assessed by a phosphorylation-dependent mobility shift. Moreover, the activity of Wnt-5a on dishevelled was blocked by pre-treatment with acyl protein thioesterase-1, indicating that palmitoylation of Wnt-5a is necessary for its function. Thus, our results suggest that Wnt-3a and Wnt-5a, respectively, activate canonical and non-canonical Wnt signalling pathways in ventral midbrain dopaminergic cells. Furthermore, we identify dishevelled as a key player in transducing both Wnt canonical and non-canonical signals in dopaminergic cells.

Classical Embryological Studies and Modern Genetic Analysis of Midbrain and Cerebellum Development

The brain is a remarkably complex anatomical structure that contains a diverse array of subdivisions, cell types, and synaptic connections. It is equally extraordinary in its physiological properties, as it constantly evaluates and integrates external stimuli as well as controls a complicated internal environment. The brain can be divided into three primary broad regions: the forebrain, midbrain (Mb), and hindbrain (Hb), each of which contain further subdivisions. The regions considered in this chapter are the Mb and most-anterior Hb (Mb/aHb), which are derived from the mesencephalon (mes) and rhombomere 1 (r1), respectively. The dorsal Mb consists of the laminated superior colliculus and the globular inferior colliculus (Fig. 1A and B), which modulate visual and auditory stimuli, respectively. The dorsal component of the aHb is the highly foliated cerebellum (Cb), which is primarily attributed to controlling motor skills (Fig. 1A and B). In contrast, the ventral Mb/aHb (Fig. 1B) consists of distinct clusters of neurons that together comprise a network of nuclei and projections-notably, the Mb dopaminergic and Hb serotonergic and Mb/aHb cholinergic neurons (Fig. 1G and H), which modulate a collection of behaviors, including movement, arousal, feeding, wakefulness, and emotion. Historically, the dorsal Mb and Cb have been studied using the chick as a model system because of the ease of performing both cell labeling and tissue transplants in the embryo in ovo; currently DNA electroporation techniques are also used. More recently the mouse has emerged as a powerful genetic system with numerous advantages to study events underpinning Mb/aHb development. There is a diverse array of spontaneous mutants with both Mb- and Cb-related phenotypes. In addition, numerous gene functions have been enumerated in mouse, gene expression is similar across vertebrates, and powerful genetic tools have been developed. Finally, additional insight into Mb/aHb function has been gained from studies of genetic diseases, such as Parkinson's disease, schizophrenia, cancer, and Dandy Walker syndrome, that afflict the Mb/aHb in humans and have genetic counterparts in mouse. Accordingly, this chapter discusses a spectrum of experiments, including classic embryology, in vitro assays, sophisticated genetic methods, and human diseases. We begin with an overview of Mb and aHb anatomy and physiology and mes/r1 gene expression patterns. We then provide a summary of fate-mapping studies that collectively demonstrate the complex cell behaviors that occur while the Mb and aHb primordia are established during embryogenesis and discuss the integration of both anterior-posterior (A-P) and dorsal-ventral (D-V) patterning. Finally, we describe some aspects of postnatal development and some of the insights gained from human diseases.

Neurodevelopment, neuroplasticity, and new genes for schizophrenia

Schizophrenia is a complex, debilitating neuropsychiatric disorder. Epidemiological, clinical, neuropsychological, and neurophysiological studies have provided substantial evidence that abnormalities in brain development and ongoing neuroplasticity play important roles in the pathogenesis of the disorder. Complementing these clinical studies, a range of cytoarchitectural, morphometric, ultrastructural, immunochemical, and gene expression methods have been applied in investigations of postmortem brain tissues to characterize the cellular and molecular profile of putative developmental and plastic abnormalities in schizophrenia. While findings have been diverse and many are in need of replication, investigations focusing on higher cortical and limbic brain regions are increasingly demonstrating abnormalities in the structural and molecular integrity of the synaptic complex as well as glutamate-related receptors and signal transduction pathways that play critical roles in brain development, synaptogenesis, and synaptic plasticity. Most exciting have been recent associations of schizophrenia with specific genes, such as neuregulin-1, dysbindin-1, and AKT-1, which are vital to synaptic development, neurotransmission, and plasticity.

Wnt11 facilitates embryonic stem cell differentiation to Nkx2.5-positive cardiomyocytes

Wnt signaling plays a crucial role in the control of morphogenesis in several tissues. Herein, we describe the role of Wnt11 during cardiac differentiation of embryonic stem cells. First, we examined the expression profile of Wnt11 during the course of differentiation in embryoid bodies, and then compared its expression in retinoic acid-treated embryoid bodies with that in untreated. In differentiating embryoid bodies, Wnt11 expression rose along with that of Nkx2.5 expression and continued to increase. When the embryoid bodies were treated with retinoic acid, Wnt11 expression decreased in parallel with the decreased expression of cardiac genes. Further, treatment of embryoid bodies with medium containing Wnt11 increased the expression of cardiac marker genes. Based on these results, we propose that Wnt11 plays an important role for cardiac development by embryoid bodies, and may be a key regulator of cardiac muscle cell proliferation and differentiation during heart development.

Divide et Impera – the midbrain–hindbrain boundary and its organizer

The midbrain-hindbrain organizer (MHO) is a signalling centre that orchestrates development of the mesencephalic and anterior metencephalic primordia. In recent years, details have been revealed about the molecular nature of these signals, their transmission and reception, and the regulatory processes associated with MHO function. This article reviews recent progress in understanding the genetic and molecular components of the MHO, and how they synergize to control brain development.

GSK-3β inhibition/β-catenin stabilization in ventral midbrain precursors increases differentiation into dopamine neurons

Wnts are important regulators of dopamine (DA) neuron differentiation in the developing ventral mesencephalon and could thus serve as potential tools in the treatment of Parkinson's disease. In this study, we investigate whether established intracellular Wnt signalling components could modulate the development of DA neurons. Two chemical inhibitors of glycogen synthase kinase (GSK)-3β, indirubin-3-monoxime and kenpaullone, were found to increase neuronal differentiation in ventral mesencephalon precursor cultures. In addition, the GSK-3β-specific inhibitor kenpaullone increased the size of the DA neuron population through conversion of precursors expressing the orphan nuclear receptor-related factor 1 into tyrosine hydroxylase positive neurons, thereby mimicking an effect of Wnts. We show that GSK-3β inhibitors stabilized β-catenin and that overexpression of β-catenin in ventral mesencephalic precursors resulted in increased DA differentiation. The three- to fivefold increase in DA differentiation of precursor cells by GSK-3β inhibitors suggests that such compounds could be used to improve stem/precursor cell therapy approaches in Parkinson's disease.

Fibroblast growth factor-20 promotes the differentiation of Nurr1-overexpressing neural stem cells into tyrosine hydroxylase-positive neurons

Stem cells are currently considered as alternative cell resources for restorative transplantation strategies in Parkinson's disease. However, the mechanisms that induce differentiation of a stem cell toward the dopaminergic phenotype are still partly unknown thus hampering the production of dopaminergic neurons from stem cells. In the past, FGF-20 has been found to promote the survival of ventral mesencephalic (VM) dopaminergic (DA) neurons in culture. We hereby provide evidence that FGF-20, a growth factor of the FGF family, is expressed in the adult and 6-OHDA-lesioned striatum and substantia nigra, but is not expressed by VM glia or DA neurons, suggesting that FGF-20 may work on DA neurons in a paracrine- or target-derived manner. We also found that co-culture of Nurr1-NSCs with Schwann cells overexpressing FGF-20 induced the acquisition of a neuronal morphology by the NSCs and the expression of tyrosine hydroxylase (TH) as assessed by immunocytochemistry, cell ELISA, and Western blot analysis. RT-PCR showed, that both, Schwann cells and Nurr1-NSCs (differentiated or not), expressed the FGF-1 receptor suggesting that both direct and indirect actions of FGF-20 are possible. We show that differentiated Nurr1 cells retained both neuronal morphology and TH expression after transplantation into the striatum of 6-OHDA-lesioned postnatal or adult rats, but that neuritogenesis was only observed after postnatal grafts. Thus, our results suggest that FGF-20 promotes the differentiation of Nurr1-NSCs into TH-positive neurons and that additional factors are required for the efficient differentiation of DA neurons in the adult brain.

WNT and beta-catenin signalling: diseases and therapies

WNT signalling has been studied primarily in developing embryos, in which cells respond to WNTs in a context-dependent manner through changes in survival and proliferation, cell fate and movement. But WNTs also have important functions in adults, and aberrant signalling by WNT pathways is linked to a range of diseases, most notably cancer. What is the full range of diseases that involve WNT pathways? Can inhibition of WNT signalling form the basis of an effective therapy for some cancers? Could activation of WNT signalling provide new therapies for other clinical conditions? Finally, on the basis of recent experiments, might WNTs normally participate in self-renewal, proliferation or differentiation of stem cells? If so, altering WNT signalling might be beneficial to the use of stem cells for therapeutic means.

Mammalian Ryk Is a Wnt Coreceptor Required for Stimulation of Neurite Outgrowth

The Ryk receptor belongs to the atypical receptor tyrosine kinase family. It is a new member of the family of Wnt receptor proteins. However, the molecular mechanisms by which the Ryk receptor functions remain unknown. Here, we report that mammalian Ryk, unlike the Drosophila Ryk homolog Derailed, functions as a coreceptor along with Frizzled for Wnt ligands. Ryk also binds to Dishevelled, through which it activates the canonical Wnt pathway, providing a link between Wnt and Dishevelled. Transgenic mice expressing Ryk siRNA exhibit defects in axon guidance, and Ryk is required for neurite outgrowth induced by Wnt-3a and in the activation of T cell factor (TCF) induced by Wnt-1. Thus, Ryk appears to play a crucial role in Wnt-mediated signaling.

Induction and specification of midbrain dopaminergic cells: focus on SHH, FGF8, and TGF-?

Cell-fate decisions along the dorsoventral and anterior-posterior axis of the neural tube are dictated by factors from signaling and organizing centers. According to the prevailing notion, the formation of mesencephalic dopaminergic neurons is directed by diffusable signals from the notochord, floor plate, and isthmic organizer. Sonic hedgehog (Shh), secreted by the notochord and floor plate, and fibroblast growth factor (FGF) 8, secreted by the isthmus, are thought to be key molecules involved in the development of midbrain dopaminergic neurons. During the last decade, the introduction of elegant explant culture systems and the generation of transgenic and mutant mice have greatly contributed to a better understanding of the molecular signals that direct the induction and specification of midbrain dopaminergic neurons. In this context, experimental evidence has challenged the dominant roles of Shh and FGF8 in dopaminergic neuron development. Additional molecules have been identified as being required for the generation of mesencephalic dopaminergic neurons, particularly members of the transforming growth factor beta superfamily.

The Drosophila Wnt5 protein mediates selective axon fasciculation in the embryonic central nervous system

The decision of whether and where to cross the midline, an evolutionarily conserved line of bilateral symmetry in the central nervous system, is the first task for many newly extending axons. We show that Wnt5, a member of the conserved Wnt secreted glycoprotein family, is required for the formation of the anterior of the two midline-crossing commissures present in each Drosophila hemisegment. Initial path finding of pioneering neurons across the midline in both commissures is normal in wnt5 mutant embryos; however, the subsequent separation of the early midline-crossing axons into two distinct commissures does not occur. The majority of the follower axons that normally cross the midline in the anterior commissure fail to do so, remaining tightly associated near their cell bodies, or projecting inappropriately across the midline in between the commissures. The lateral and intermediate longitudinal pathways also fail to form correctly, similarly reflecting earlier failures in pathway defasciculation. Panneural expression of Wnt5 in a wnt5 mutant background rescues both the commissural and longitudinal defects. We show that the Wnt5 protein is predominantly present on posterior commissural axons and at a low level on the anterior commissure and longitudinal projections. Finally, we demonstrate that transcriptional repression of wnt5 in AC neurons by the recently described Wnt5 receptor, Derailed, contributes to this largely posterior commissural localization of Wnt5 protein.